1. Field of the Invention
The present invention relates to an optical modulation device which receives a light beam and modulates its intensity, and more particularly to an optical modulation device adapted to allow control of the chirp characteristics of the output beam.
2. Background Art
The three types of optical modulators capable of high-speed modulation (10 Gbit/s or more) which have been used in practical applications are: (1) the lithium niobate (LiNbO3) Mach-Zehnder optical modulator, (2) the semiconductor Mach-Zehnder optical modulator, and (3) the electro absorption optical modulator. Lithium niobate Mach-Zehnder optical modulators are most widely used as modulation light sources in optical transponders, since their performance varies only a little with temperature and wavelength variations and they have stable chirp characteristics.
In a lithium niobate Mach-Zehnder optical modulator, the incident light beam is divided equally by an optical splitter into two beams which are then passed through their respective waveguides. At that time, modulating signals of equal amplitude but 180° out of phase are respectively applied to these waveguides to change their refractive indices and thereby change the phases of the waveguide beams by ±90°, respectively. The waveguide beams are then combined by an optical combiner and output from the modulator, thus converting the phase modulation into intensity modulation.
Lithium niobate Mach-Zehnder optical modulators typically have a waveguide length of 30-50 mm; that is, optical semiconductor devices incorporating this type of optical modulator must be as much as 50-100 mm in length. Although prior art optical transponders (dimensioned 5 inches by 7 inches, or 4.5 inches by 3.5 inches) have a space for accommodating such an optical semiconductor device(s), there is no such space available in XFP (10 Gigabit Form Factor Pluggable) optical transceivers, which have been recently used in response to the decreasing size of optical communications devices. It is not possible to sufficiently reduce the size of lithium niobate Mach-Zehnder optical modulators, since application of a voltage to LiNbO3 results in only a small change in its refractive index (the actual amount of change being determined by the material constants). As a result, this type of optical modulator must have a length on the order of a few tens of millimeters or more (as described above) to introduce a 90° phase change in the beams traveling through its waveguides.
In the case of a semiconductor Mach-Zehnder optical modulator, on the other hand, the modulator can cause ±90° phase changes in the beams propagating through its semiconductor optical waveguides even if the waveguides are as short as approximately a few millimeters in length, provided that they have a band gap wavelength approximately 100 nm shorter than the wavelength of the incident light. Such semiconductor Mach-Zehnder optical modulators have proven to function satisfactorily. Therefore, the size of semiconductor Mach-Zehnder optical modulators can be reduced, making them suitable for use in XFP optical transceivers. Furthermore, a semiconductor Mach-Zehnder optical modulator may be formed from a material used to form an optical communications laser (e.g., InGaAsP on an InP substrate). This enables the optical modulator to be integrally and monolithically formed with the optical communications laser, resulting in a simplified optical system and hence reduced cost. It should be noted that the performance of semiconductor Mach-Zehnder optical modulators is more susceptible to temperature and wavelength variations than the performance of lithium niobate Mach-Zehnder optical modulators but less susceptible than the performance of electro absorption optical modulators. Therefore, semiconductor Mach-Zehnder optical modulators are a promising optical modulator that can be combined with a variable wavelength laser to provide a next-generation small size variable wavelength modulation light source.
However, the length of semiconductor Mach-Zehnder optical modulators (approximately a few millimeters) is still too large to form them in a sufficient quantity on a compound semiconductor wafer, resulting in increased manufacturing cost. (For example, InP wafers are 2-3 inches in diameter.) On the other hand, electro absorption optical modulators can be approximately 0.2 mm long, with an extinction ratio of approximately 10 dB, for example. Furthermore, they can be easily monolithically integrated with a semiconductor laser and are often used in fixed wavelength XFP transceivers.